1. Field of the Invention
The present invention relates to a bearing assembly having an absolute encoder built therein, which has a capability of detecting the absolute angle of rotation and can be employed in various equipments such as articulates of a robot or articulated manipulator.
2. Description of the Prior Art
In view of the easiness to assemble compact, a rolling bearing assembly having a rotation sensor built therein is currently placed in the market, an example of which is shown in FIG. 11 of the accompanying drawings. The rolling bearing assembly 51 in FIG. 11 includes an inner race 52, which is a rotatable raceway member, an outer race 53, which is a stationary raceway member and encloses the inner race 52 with a cylindrical bearing space defined therebetween, and a circumferential row of rolling elements 54 rollingly retained by a roller retainer 55 and interposed between the inner and outer races 52 and 53.
An annular magnetic encoder 56 is secured to one end of the inner race 52 for rotation together therewith and is employed in the form of, for example, an annular rubber magnet having a plurality of opposite magnetic N and S poles alternating with each other in a direction circumferentially thereof. Cooperable with the annular magnetic encoder 56 is a magnetic sensor 57, which is in the form of, for example, a Hall element and secured to a corresponding end of the outer race 53 in face-to-face relation with the magnetic encoder 56. The magnetic sensor 57 is resin molded or encapsulated in a resinous casing 58, which is in turn fixedly mounted on the outer race 53 by means of a metallic casing 59.
In the rolling bearing assembly 51 of the above structure, the magnetic sensor 57 detects alternating change in magnetic poles of the magnetic encoder 56, as shown in FIG. 12, during rotation of the inner race 52 and subsequently outputs a train of incremental pulses, as shown in FIG. 13, which is descriptive of the number of revolutions, or the rotational speed, of the inner race 52.
It has, however, been found that with the rotation sensor of the structure described above, although the incremental pulse signal descriptive of the rotational speed of the inner race 52 is obtained, the rotation sensor is unable to provide the absolute angle of rotation of the inner race 52, unless the power-on initialization of the rotation sensor is carried out before counting of the pulses starts.
In order to alleviate the above discussed inconvenience, the Japanese Laid-open Patent Publication No. 2004-4028, published Jan. 8, 2004, for example, discloses, as shown in FIG. 14, a bearing assembly with a built-in absolute encoder, in which a radial type to-be-detected element 61, mounted on an inner race, has a magnetic characteristic varying in a substantially sinusoidal waveform having a cycle matching with one complete rotation of the inner race. A magnetic sensor unit 60 for detecting change in magnetism of the to-be-detected element 61 is comprised of two magnetic sensors 60A and 60B arranged at respective locations radially outwardly of the to-be-detected element 61 and spaced a predetermined angular distance from each other in a direction circumferentially of the to-be-detected element 61.
According to the above structure, since the magnetic characteristic of the to-be-detected element 61 is so designed as to vary with each cycle matching with one complete rotation of the inner race, the magnetic sensor unit 60 can easily output a signal indicative of the absolute angle of rotation. Also, when an output indicative of the difference between respective outputs from the two magnetic sensors 60A and 60B is subjected to rectangular pulse shaping, a rectangular signal of a cycle matching with one complete rotation of the inner race can be obtained as an origin signal, i.e., a signal indicative of the original angular position of the inner race.
Alternatively, a sinusoidal output generated from one of the two magnetic sensors 60A and 60B is compared with a center voltage intermediate of the amplitude of such sinusoidal output to provide a rectangular signal, which can be used as the origin signal.
However, the absolute encoder disclosed in the above discussed patent publication is unable to provide the origin signal of a high accuracy since it tends to be adversely affected by variation in threshold value between the magnetic sensors 60A and 60B and/or decrease in magnetism of the to-be-detected element 61 under the influence of temperatures. As such, where the repeatability of the origin signal is strictly required, it is necessary to employ, in addition to a combination of the to-be-detected element 61 and its cooperating magnetic sensor unit 60 associated with the detection of the absolute angle of rotation, an additional combination of a to-be-detected element and an additional magnetic sensor unit associated with the detection of the original position, so that an origin signal can be generated.
In order to alleviate the foregoing problems and inconveniences, the previously discussed patent publication also discloses an absolute encoder of an alternative structure, in which as shown in FIG. 15, a combination of a to-be-detected element 67 and a magnetic sensor unit 68 for the detection of the absolute angle of rotation and a combination of a to-be-detected element 87 and a magnetic sensor unit 88 for the detection of the original position are employed.
Referring to FIG. 15, the inner race 52 carries not only the first to-be-detected element 67 associated with the detection of the absolute angle of rotation, but also the second to-be-detected element 87 associated with the detection of the original position and, on the other hand, the outer race 53 carries not only the first magnetic sensor unit 68 cooperable with the to-be-detected element 67, but also the second magnetic sensor unit 88 cooperable with the to-be-detected element 87. The first to-be-detected element 67 for the detection of the absolute angle of rotation is of a radial type, in which as shown in FIG. 16A in a transverse sectional representation, the pattern of magnetization varies in a substantially sinusoidal waveform with a cycle matching with one complete rotation of the inner race 52. The first magnetic sensor unit 68 for the detection of the absolute angle of rotation is comprised of two magnetic sensors 68A and 68B arranged at respective locations radially outwardly of the to-be-detected element 67 and spaced a predetermined angular distance, for example, 90° from each other in a direction circumferentially of the to-be-detected element 67. Using respective outputs from the two magnetic sensors 68A and 68B, it is possible to determine the quadrant and, hence, the absolute angle of rotation of the inner race 52 can be indicated.
On the other hand, the second to-be-detected element 87 for the detection of the original position is also of a radial type, in which as shown in FIG. 16B in a transverse sectional representation, the pattern of magnetization is such that a pair of magnetic N and S poles is created in a direction circumferentially thereof or, alternatively, a single pole, for example, S pole is created in a direction circumferentially thereof. The second magnetic sensor unit 88 for the detection of the original position is comprised of a single magnetic sensor of a latch type or a switch type capable of generating an output signal corresponding to the magnetic flux density. As the inner race 52 rotates, the second magnetic sensor unit 88 provides the origin signal in terms of change in magnetism of the pair of N and S poles of the second to-be-detected element 87.
It has, however, been found that if the distance D between the first to-be-detected element 67 and the second to-be-detected element 87, as well as the distance D′ between the first magnetic sensor unit 68 and the second magnetic sensor unit 88, is small as shown in FIG. 17A, respective leakage fluxes from the first and second to-be-detected elements 67 and 87 will interfere with each other. Where the second to-be-detected element 87 is magnetized to have a single S pole, the magnetic characteristic of the first to-be-detected element 67 in a direction radially thereof will be such as shown in FIG. 17B and the magnetic characteristic of the second to-be-detected element 87 in a direction radially thereof will be such as shown in FIG. 17C.
Under these conditions, a considerable error tends to occur in the absolute angle of rotation represented by the output signals from the magnetic sensors 68A and 68B forming respective parts of the first magnetic sensor unit 68. Also, the output from the second magnetic sensor unit 88 associated with the detection of the original position is latched (or switched) in the vicinity of a region where the magnetic flux of the to-be-detected element 87 decreases to zero, not in the vicinity of the single S pole where the magnetic flux of the to-be-detected element 87 varies considerably and, therefore, the accuracy of the origin detection signal tends to be lowered.